It is common to complete oil and gas wells by perforating the well casing and surrounding producing formation. Typically a perforating apparatus is attached to the end of a tubing string and lowered adjacent the producing formation. The perforating apparatus then is fired, and the well produces through the tubing string. One example of a tubing string-conveyed perforating apparatus can be found in U.S. Pat. No. 4,523,643, issued to McGlothen and assigned to the assignee of the present invention, which is incorporated herein by reference.
The perforating apparatus typically has been fired electronically or by dropping a detonating bar down the tubing string to strike a firing head. Neither method has been entirely satisfactory. Electronic firing systems require care in connecting and running the system down the tubing. Electronic firing systems also can be activated accidentally by stray electrical currents and can be short-circuited by moisture. A perforating apparatus for use with a detonating bar contains a charge designed to explode when struck, making such a perforating apparatus dangerous to handle. Further, it may not be possible to use a detonating bar in a highly deviated well.
Various pressure firing systems have been developed to overcome some of the deficiencies of the electronic and bar firing systems. In one early embodiment a fluid or gas pressure is applied through the tubing string to a pressure firing system. The pressure forces a firing pin into contact with a detonator, thus firing the perforating apparatus. One type of pressure firing system is illustrated in U.S. Pat. No. 2,304,408, issued to Holifiled. Pressure firing systems of this type are also dangerous to handle because they may fire upon the application of any unintended pressure.
Firing systems have been developed recently that are actuated by directed pressure, minimizing the risk of accidental firing. In these firing systems pressurized fluid in the tubing is directed inside the firing apparatus where it works against a piston. Sufficient force caused by the fluid pressure against the piston shears a coupling and in turn releases a spring-loaded firing pin. This system is exemplified in U.S. Pat. No. 4,770,246, issued to Ward, and U.S. Pat. No. 4,886,127, issued to Ricles et al., both assigned to the assignee of the present invention, which are incorporated herein by reference. This type of system functions well but has the added complexity of combining the pressure actuated mechanism with the spring-loaded firing mechanism.
These and other disadvantages are overcome by the present invention. The present invention discloses a firing apparatus in which the wellbore pressure serves both to actuate the apparatus and to fire the perforating apparatus.